1. Field Of The Invention
This invention relates to thermosetting coating compositions in powder form. More particularly this invention relates to heat hardenable powdered coating compositions comprising carboxyl group containing polyesters, polyepoxide compounds and cross-linking catalyst where the cross-linking catalyst is a titanium or zirconium chelate which is solid at room temperature. The heat hardenable powdered coating composition of the present invention can also include conventional pigments and fillers as well as additives and a adjuvants such as leveling agents and the like.
2. Discussion Of The Prior Art
Thermosetting coating compositions are known from German Offenlegungsschrift No. 21 63 962, which contain the components named above. The polyester resins containing carboxyl groups described therein are prepared with difficulty by a multi-step process. Compounds are described as polyepoxide compounds which contain at least two epoxide groups, such as, for example, diglycidylterephthalate, triglycidylisocyanurate or polyglycidyl ethers of 0-cresol-formaldehyde novolacs. In this case tris-(2,4,6-dimethylaminomethyl)-phenol (DMP) is used as catalyst. There are difficulties involved, however, in incorporating the liquid DMP into powdered varnish compositions, using, for example, a Rhon wheel mixer. The preparation of "master batches," as they are called, is also difficult. If DMP is added to a melt of the polyester resin, the latter becomes considerably discolored.
German Offenlegungsschrift 23 28 012 relates to coating substances in powder form composed of polyesters containing free carboxyl groups or succinic acid, and cross-linking agents which react with carboxyl groups at elevated temperatures. The acid polyesters claimed therein are prepared by the reaction of hydroxyl-group-containing polyesters with succinic acid anhydride, in mixture, if desired, with other dicarboxylic acid anhydrides.
In addition to the previously mentioned solid polyepoxides made from bisphenol A and epichlorhydrin, the cross-linking agents named are various glycidyl esters of di-, tri- and tetracarboxylic acids as well as glycidyl derivatives of hydantoin and/or polyoxazolines. The catalysts that can be added are, for example, tertiary amines and their salts such as benzyldimethylamine, metal salts of organic acids such as tin octoate or bismuth salicylate, or also imidazoles.
The catalytic activity of the compounds named differs considerably, and, especially in the case of the use of polyepoxide resins made from bisphenol A and epichlorhydrin, they are unsatisfactory. At low setting temperatures below 200.degree. C., insufficiently cross-linked films are obtained; higher temperatures than 200.degree. C. are uneconomical, and also the varnish films can discolor. Where these substances are liquid, the disadvantages are the same as they are with DMP.